1. Field of the Invention
The present invention relates to an image processing apparatus capable of outputting high gradation and high quality image output information while reducing a substantial quantifying bit number of the image output information.
2. Related Background Art
A photoelectric conversion element which has a one-line structure and is composed of minute cells which can read at a resolution of several lines/mm to several tens of lines/mm, such as a CCD sensor or a contact sensor is used for an image reading apparatus which receives an original and reads an image of the original, such as a copying machine or a scanner.
In general, reading is performed in the main scanning direction by electrical scanning of the sensor itself and reading is performed in the sub scanning direction by moving the entire photoelectric conversion element relative to the original.
When high grade reading is performed by the above-mentioned image reading apparatus, it is necessary to obtain faithful information on the original having high gradation. In this case, it is important to recognize the gradation according to a visual characteristic of the naked eye. That is, it is necessary to read a difference of the gradation in a range of a reflectance or a transmittance, which is easy to visually recognize the difference of the gradation.
It has been known that the visually recognized gradation is not in a linear relationship with the amount of reflection light (the amount of transmission light) on the original but substantially proportional to the amount D (D=−log10r, r: reflectance or transmittance) which is called “a density”. In other words, when the amount of light becomes smaller (that is, when the gradation is closer to black), a slight difference in the reflectance or the transmittance can be visually recognized.
However, an output characteristic of the sensor is generally proportional to the amount of input light. Therefore, it is required that gradation information on the black side has higher precision than the gradation information on the white side.
In the case of image information converted into a digital signal by A/D conversion, for example, when the image information is eight bits, the gradation information on the black side is roughly quantified, so that the gradation information does not substantially reach the eight bits. Thus, the high precision is required.
The following apparatus has been known as an apparatus for improving a quantifying resolution on the black side in the output characteristic of the sensor (for example, Japanese Patent Application Laid-Open No. H05-227437). In the apparatus, a first A/D conversion circuit is provided for converting the output of a photoelectric conversion element into a digital signal. An amplification circuit is provided for amplifying the output of the photoelectric conversion element by an amplification factor of 2n at the polarity in which an output becomes zero at the time when the amount of input light is zero. A second A/D conversion circuit is provided for converting the output of the amplification circuit into a digital signal. A selecting and bit add-in circuit is provided for adding predetermined bits to a high order or low order of each of the outputs of the first and second A/D conversion circuits and selecting an added output.
According to the apparatus, the quantifying resolution on the most black side of the original can be improved. Thus, the high gradation image reading according to the visual characteristic can be performed.
FIG. 6 is a block diagram showing a conventional image processing apparatus 600.
A CCD sensor 601 serving as the photoelectric conversion element is connected with a sample hold circuit 602. The sample hold circuit 602 is connected with a sample hold circuit 603 and a peak hold circuit 604. Each of the sample hold circuits 602 and 603 and the peak hold circuit 604 are connected with a first A/D conversion circuit 605 and an amplification circuit 606 in a subsequent stage.
The amplification circuit 606 is connected with a second A/D conversion circuit 608 through a limit circuit 607 in a subsequent stage. The first A/D conversion circuit 605 and the second A/D conversion circuit 608 are connected with a selecting and bit add-in circuit 609 in a subsequent stage.
The selecting and bit add-in circuit 609 is connected with a shading correcting circuit 611 through a dark output correcting circuit 610 in a subsequent stage. The dark output correcting circuit 610 includes a correction memory 612 and the shading correcting circuit 611 includes a correction memory 613. The shading correcting circuit 611 is connected with an output terminal 614 in a subsequent stage. The first A/D conversion circuit 605 converts the output of the CCD sensor 601 into a digital signal. The amplification circuit 606 amplifies the output of the CCD sensor 601 by an amplification factor of 2n at the polarity in which an output becomes zero at the time when the amount of input light is zero.
The second A/D conversion circuit 608 converts the output of the amplification circuit 606 into a digital signal. The selecting and bit add-in circuit 609 adds predetermined bits to a high order or low order of each of the outputs of the first and second A/D conversion circuits 605 and 608 and then selects an added output.
Next, the operation of the conventional image processing apparatus 600 will be described.
First, general A/D conversion is performed on an analog signal outputted from the CCD sensor 601 over all ranges by the first A/D conversion circuit 605.
Simultaneously, the output signal of the sample hold circuit 602 is led to the amplification circuit 606 and amplified by an amplification factor of n-power of two (for example, an amplification factor of 16) based on a DS (E) value. The output of the amplification circuit 606 is led to the limit circuit 607 and limited by a smaller suitable value than VrefB. The output signal from the limit circuit 607 is inputted to the second A/D conversion circuit 608.
References VrefT and VrefB of the second A/D conversion circuit 608 are identical to the references of the first A/D conversion circuit 605. Therefore, of all ranges of the output of the CCD sensor 601, in a 1/16 range which is closest to the black side, the quantifying resolution of 16 times can be obtained. That is, it is possible to obtain an A/D conversion output in which the number of bits is larger than that of the output signal from the first A/D conversion circuit 605 by four bits on a low order side.
FIG. 7 shows a state of conversion processing in the conventional image processing apparatus 600.
Here, “a main A/D conversion circuit” corresponds to the first A/D conversion circuit 605 shown in FIG. 6 and “a sub A/D conversion circuit” corresponds to the second A/D conversion circuit 608 shown in FIG. 6. Each A/D conversion circuit is an eight-bit A/D conversion circuit.
An upper side region shown in FIG. 7 indicates an output code at input on the white side. As compared with 16 different output codes from a smaller one in the output signal of the main A/D conversion circuit, an output code of the sub A/D conversion circuit further has a resolution of 16 times.
Therefore, in order that the output signal of the A/D conversion circuit have 12 bits in total, when the output signal of the sub A/D conversion circuit has all “1's”, low four bits are further added to the output signal of the main A/D conversion circuit and predetermined values (for example, all “0's”) are provided for the low four bits to obtain 12 bits. When the output signal of the sub A/D conversion circuit has information other than all “1's”, high four bits, each of which is “0”, are further added to the output signal of the sub A/D conversion circuit to obtain 12 bits.
Thus, a characteristic that the quantifying resolution on the black side of the original is higher than a general quantifying resolution is obtained. Note that the operation in which the number of bits of the output signal is increased to 12 bits in total is executed by the selecting and bit add-in circuit 609.
After the resultant signal is outputted from the selecting and bit add-in circuit 609, a dark output changed for each pixel of the CCD sensor 601 is corrected by the dark output correcting circuit 610. In other words, a content stored in advance in the correction memory 612 is read for each of the pixels and computed to correct the dark output changed for each pixel of the CCD sensor 601.
After that, the shading correcting circuit 611 reads correction data stored in advance in the correction memory 613 for each of the pixels and computes to correct a variation in sensitivity for each of the pixels of the CCD sensor 601, an intensity distribution of an illumination system, and the like. An image reading output can be taken from the output terminal 614.
According to the above-mentioned series of processings, the quantifying resolution on the most black side of the original can be increased by 16 times. Therefore, it is possible to perform the high gradation image reading according to the visual characteristic of the naked eye.
In the above-mentioned conventional example, in order to increases the quantifying resolution on the black side in the output characteristic of the sensor, it is necessary to provide the first A/D conversion circuit for converting the output of the photoelectric conversion element into the digital signal, the amplification circuit for amplifying the output of the photoelectric conversion element by the amplification factor of 2n at the polarity in which the output signal becomes zero at the time when the amount of input light is zero, the second A/D conversion circuit for converting the output signal of the amplification circuit into the digital signal, and the selecting and bit add-in circuit.
In addition, in the conventional example, the correction memories are provided in the subsequent stage of the selecting and bit add-in circuit. Therefore, it is required that the correction memories have a bit width corresponding to the quantifying resolution.
The conventional example has a problem in that a circuit scale and a memory capacity are increased in order to realize the high gradation image reading according to the visual characteristic of the naked eye by improving the precision of the quantifying resolution on the black side. Thus, the conventional example tends to increase a cost of the apparatus.